When a voltage is impressed to a conventional TN mode TFT display or a conventional STN display, liquid crystal molecules rotate to extend in the direction perpendicular to the substrate. When such liquid crystal molecules rise diagonally (i.e., such that they extend perpendicular to the substrate), there arise problems in that the optical properties of the liquid crystal molecules differ depending on the angle from which the liquid crystal panel is viewed, and in that the view angle is narrow.
As systems for realizing a wide view angle, the In-Plane-Switching (IPS) system characterized in the formation of comb style electrodes on one substrate (G. Baur, Freiburger Arbeistagung Flussigkristalle, Abstract No. 22 (1993), M. Oh-e, et al., ASIA DISPLAY '95, 577 (1995)), and the Vertically Aligned (VA) system (K. Ohmuro, et al., SID 97 DIGEST, 845 (1997)) have attracted attention, and have been put into practical use.
In the IPS system, since liquid crystal molecules rotate within the surface plane of the glass substrate, the view angle is greatly widened. In the VA system, liquid crystal molecules rotate from the vertical direction to the horizontal direction to the substrate, and a wide view angle is realized by controlling orientation in a kind of multi-domain system.
However, as compared to CRTs, these display systems still have problems, and there are the demands for improvement of response time, improvement of contrast, and decrease of driving voltage.
The liquid crystalline compound employed in IPS and VA systems must have the large negative value of dielectric anisotropy (Δε) for its display properties, and a low viscosity for improving response time. Since active matrix driving is utilized as the driving method, the compound must have a high voltage holding ratio (V.H.R.) in order to improve contrast.
Various compounds have been known to have negative values of dielectric anisotropy, and the following compounds (a) and (b) are disclosed in Japanese Patent Application Laid Open No. 2-4725 and Japanese-translated PCT Patent Application Laid-open No. 2-503441:
Where, R′ represents an alkyl group or an alkoxy group.
Although each of the above liquid crystalline compounds, (a) and (b), has 2,3-difluoro-1,4-phenylene group as part of its structure, and has a negative Δε, |Δε| is not sufficiently large and viscosity is high. Furthermore, mutually good solubility its with other liquid crystalline compounds, especially mutually good solubility at very low temperature, is not sufficiently high, and a liquid crystal composition containing such compounds is so unstable that low-temperature storage results in precipitation of crystals or appearance of a smectic phase.
In order to solve the problems involved in the prior art techniques, an object of the present invention is to provide a liquid crystalline compound having a wide temperature range within which a liquid crystal phase exists (hereinafter called a “liquid crystal phase temperature range”), a large negative Δε, low viscosity, and mutually good solubility at low temperature, which can contribute to improving response time and contrast and lowering driving voltage in IPS and VA systems and can contribute to improving steepness of the V-T (voltage-transmissivity) curve and contrast by increasing the K33/K11 value in the STN system; a liquid crystal composition containing such liquid crystalline compounds, and a liquid crystal display device constituted from such a liquid crystal composition.